Single faced corrugated paper board is very well known and is used extensively in industry, especially in packaging where it is used as protective padding and wrapping. It is also used as a basic component in the production of many structural objects such as boxes, panels, pallets etc.
Sheets of single faced corrugated paper board can be bonded together to form multilayer corrugated paper boards of various thickness and strength. One reason for the wide use of such paper board is its relatively light weight, rigidity and strength.
Single faced corrugated paper board is made by bonding a fluted sheet, typically of corrugating medium (generally recycled paper) paper, onto a liner sheet. The liner sheet is typically Kraft paper although other materials may be used.
In a conventional machine for producing single faced fluted paper board, the fluted sheet is formed by passing a sheet of paper between two corrugating rollers. The two rollers are arranged such that there is an inter-meshing of the teeth at the periphery of each roller. A sheet of paper is fed between the teeth of the rollers which forces the paper into flutes as the teeth of the rollers intermesh. The fluted sheet is held in contact with the teeth on the periphery of one of the corrugating rollers, typically by a vacuum formed within the corrugating cylinder, at least until the liner is added.
A line of adhesive is applied to the crests of the flutes to bond the liner to the fluted paper. A sheet of liner paper is typically pressed onto the crests by a smooth roller so as to bond the liner with the fluted paper to form a single faced corrugated (or fluted) board.
A limiting factor in this process is the requirement to maintain contact between the fluted sheet and the liner for sufficient time for a bond to be formed. This time can be several seconds or longer at normal room temperatures and pressures. However, the time can be reduced by raising the temperature during the bonding process and by applying pressure to the join between the crest and the liner.
High pressure steam is commonly used to heat the cylinders over which the fluted paper and liner paper move, raising the temperature of the paper. Typical operating temperatures for the corrugating rollers can be around 200° C.
Pressure can be applied to the joint between the liner and each crest by a pressure roller arranged to press the liner against the glued crest. As the contact time between the pressure roller and corrugating roller (which holds the fluted sheet) is relatively short, high pressures are generally required to significantly speed up the bonding process.
With this type of arrangement (heating plus high pressure) the bonding time can be reduced to a few hundredths of a second, allowing high speed production of single faced paper board. However, one problem with this arrangement is that the high pressure exerted by the pressure roller can distort the paper and leave unsightly marks on the surface of the liner paper, reducing its value.
In some machines other devices, such as endless belts, are used to press the liner and fluted sheets together as they move around the corrugating roller. This extends the period of time pressure is applied, and therefore lower pressures can be used so as not to deform the surface of the paper board.
A typical arrangement of a machine to produce single faced paper board generally as described above is disclosed in U.S. Pat. No. 5,951,817 (Thomas).
The machinery depicted in U.S. Pat. No. 5,951,817 has a limited contact time (and hence bond time) between the fluted sheet and the liner—as evidenced by the fluted sheet/liner combination only being in contact with a third of the circumference of the corrugated wheel. Such limited contact time requires the use of a fast drying adhesive such as a starch based glue used typically in the industry. Unfortunately, starch based glues requires the application of heat (say in the order of 150° C. or so) to activate. This leads to disadvantages as discussed below.
Although the use of heat can shorten the time required to form a bond between the fluted sheet and the liner, and therefore allow high through-put, it does introduce a number of engineering issues which significantly increase the cost of the machinery and the operating costs for the process. The use of high pressure steam introduces the need for boilers, pipe work, and housing to ensure containment of the steam at all times. There is a need for additional safety measures to ensure that the steam is produced and managed correctly and that the heat produced does not provide a safety hazard during operation of the machine. Typically a heating engineer is required to operate the boiler and maintain the heating system, adding the cost of additional skilled labour, as well as potentially stopping production if such an engineer is not available.
The addition of a high pressure steam system to the basic elements of the machine inevitably leads to an increase in the size of the machine and therefore the space required for operation of the machine. The requirement for more space adds to the operating costs for production of the single faced fluted board.
Furthermore high pressure steam is highly corrosive to many materials, and those parts of the machine exposed to the steam, for example the corrugating rollers, need to be made from appropriate materials. Such materials, for example alloy steel 48 CrMo hardened to HRC 58-62 standard, are generally expensive and heavy. A machine made from such materials can require substantial support structure to maintain the structural integrity of the machine.
A problem with both methods of applying pressure (roller and belt) is that the applied pressure can spread some of the adhesive away from the contact line between the liner and the fluted sheet across the crest, thus wetting the surrounding paper. As this spread adhesive is not under pressure (once away from the contact line) and does not necessarily form a bond, further heating can be required in order to dry out the paper and superfluous (spread) adhesive. This adds to the cost of production and may limit the speed of the process.
The spread adhesive can also get transferred to the surface of the device applying the pressure (eg, roller or belt) requiring the surface of the device to be continually cleaned. Some adhesive can be transferred to the outer surface of the liner, which can cause problems during storage when rolled, as neighbouring sheets can stick together making the product unusable. Further, the adhesive can mark the surface, adversely affecting the appearance and reducing value of the corrugated board.
A further disadvantage with conventional machines is that typically the feed rollers used to feed the paper (both to be fluted and used as a liner) onto the corrugating rollers, and the corrugating rollers, are heated in order to dry out the paper prior to addition of the adhesive. This is to reduce the moisture content of the paper and to produce a rapid bonding time. The production of high pressure steam to heat the corrugating rollers and to pre-heat the paper can require a significant amount of energy.
All of the above factors can add significantly to the cost of single face fluted board made by conventional processes and machines.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicant reserves the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications may be referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.